Texas Instruments LM3671 Evaluation Board LM3671LC-1.8EV/NOPB LM3671LC-1.8EV/NOPB 数据表
产品代码
LM3671LC-1.8EV/NOPB
I
RMS
= I
OUTMAX
The worst case is when V
IN
= 2 V
OUT
(V
IN
- V
OUT
) V
OUT
L f I
OUTMAX
V
IN
r =
V
OUT
V
IN
r
2
12
1 -
+
V
OUT
V
IN
¸¸
¹
·
¨¨
©
§
¸
¹
·
¨
©
§
f
1
L
2
!
I
SAT
I
OUTMAX
+ I
RIPPLE
where I
RIPPLE
=
V
OUT
V
IN
¸
¹
·
¨
©
§
¸
¹
·
¨
©
§
V
IN
- V
OUT
SNVS294Q – NOVEMBER 2004 – REVISED NOVEMBER 2013
Inductor Selection
There are two main considerations when choosing an inductor; the inductor should not saturate, and the inductor
current ripple should be small enough to achieve the desired output voltage ripple. Different saturation current
rating specifications are followed by different manufacturers so attention must be given to details. Saturation
current ratings are typically specified at 25°C. However, ratings at the maximum ambient temperature of
application should be requested from the manufacturer. The minimum value of inductance to specify good
performance is 1.76 µH at I
current ripple should be small enough to achieve the desired output voltage ripple. Different saturation current
rating specifications are followed by different manufacturers so attention must be given to details. Saturation
current ratings are typically specified at 25°C. However, ratings at the maximum ambient temperature of
application should be requested from the manufacturer. The minimum value of inductance to specify good
performance is 1.76 µH at I
LIM
(typ.) dc current over the ambient temperature range. Shielded inductors
radiate less noise and should be preferred.
There are two methods to choose the inductor saturation current rating.
Method 1:
The saturation current should be greater than the sum of the maximum load current and the worst case average
to peak inductor current. This can be written as
to peak inductor current. This can be written as
where
•
I
RIPPLE
: average to peak inductor current
•
I
OUTMAX
: maximum load current (600 mA)
•
V
IN
: maximum input voltage in application
•
L : min inductor value including worst case tolerances (30% drop can be considered for method 1)
•
f : minimum switching frequency (1.6 Mhz)
•
V
OUT
: output voltage
(7)
Method 2:
A more conservative and recommended approach is to choose an inductor that has a saturation current rating
greater than the maximum current limit of 1150mA.
greater than the maximum current limit of 1150mA.
A 2.2 µH inductor with a saturation current rating of at least 1150 mA is recommended for most applications. The
inductor’s resistance should be less than 0.3
inductor’s resistance should be less than 0.3
Ω for good efficiency.
lists suggested inductors and
suppliers. For low-cost applications, an unshielded bobbin inductor could be considered. For noise critical
applications, a toroidal or shielded-bobbin inductor should be used. A good practice is to lay out the board with
overlapping footprints of both types for design flexibility. This allows substitution of a low-noise shielded inductor,
in the event that noise from low-cost bobbin models is unacceptable.
applications, a toroidal or shielded-bobbin inductor should be used. A good practice is to lay out the board with
overlapping footprints of both types for design flexibility. This allows substitution of a low-noise shielded inductor,
in the event that noise from low-cost bobbin models is unacceptable.
Input Capacitor Selection
A ceramic input capacitor of 4.7 µF, 6.3V is sufficient for most applications. Place the input capacitor as close as
possible to the V
possible to the V
IN
pin of the device. A larger value may be used for improved input voltage filtering. Use X7R or
X5R types; do not use Y5V. DC bias characteristics of ceramic capacitors must be considered when selecting
case sizes like 0805 and 0603. The minimum input capacitance to specify good performance is 2.2 µF at
3V dc bias; 1.5 µF at 5V dc bias including tolerances and over ambient temperature range. The input filter
capacitor supplies current to the PFET switch of the LM3671 in the first half of each cycle and reduces voltage
ripple imposed on the input power source. A ceramic capacitor’s low ESR provides the best noise filtering of the
input voltage spikes due to this rapidly changing current. Select a capacitor with sufficient ripple current rating.
The input current ripple can be calculated as:
case sizes like 0805 and 0603. The minimum input capacitance to specify good performance is 2.2 µF at
3V dc bias; 1.5 µF at 5V dc bias including tolerances and over ambient temperature range. The input filter
capacitor supplies current to the PFET switch of the LM3671 in the first half of each cycle and reduces voltage
ripple imposed on the input power source. A ceramic capacitor’s low ESR provides the best noise filtering of the
input voltage spikes due to this rapidly changing current. Select a capacitor with sufficient ripple current rating.
The input current ripple can be calculated as:
(8)
20
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